2520 coolant system cleanup

I have my tractor apart. Radiator and thermostat are out. I did this because I was running it a couple weeks ago and I turned the rpm down and jumped off to move some stuff around for a few minutes and it stalled. When I went to start it, it didn't want to start. Waited a bit and it started. 10 mins later it did it again. I put it away. next week I took it out to move some stuff. I got it warmed up and was running it at PTO speed for 15 mins. Temperature gauge never showed hot. Looked normal. I put it away and when I shut it off it ran on for a second. I decided since I had not flushed the system since I got it, I would pull off the hood and see what is what. I bought a new belt, thermostat and gasket for thermostat. What I saw was:
Radiator fins are filled with dust and oil? sludge.
Top of motor with same sludge.
Hoses look half OK.
Antifreeze looks dark.
the belt was way loose with chunks missing
Opening the petcock on the radiator yielded about 2.5 gallons of coolant, but the radiator looked full before I started. This was with the thermostat out.
So far in my shopping cart at the Deere dealer I have:
all new hoses and clamps
gasket for under the thermostat housing
radiator cap
air filter
I have an aftermarket temp gauge on order to verify the Deere gauge.
I am wondering the best way to proceed with cleaning and verifying everything. I don't see any freeze plugs, but I may just be missing them. I want to somehow confirm I don't have junk in my water jacket and figure out why the temp gage didn't show high when I suspect it was. I don't want to damage the radiator with a bad cleaning plan. Any advice or suggestions would be appreciated. I do have a coolant heater so was thinking of plugging in a hose to that connection for flushing the block.

Well for one thing the radiator drain will not drain the engine, so a gallon is probably still in the block,as for cleaning the radiator fins your best to remove the radiator and blow all you can with air then "carefully" power washing the core, if you have a very high pressure washer keep it back from the core a bit, too much pressure and to close will cut the cores up, keep the flow straight on so the fins don't get bent. A local car wash would work out okay ,, they only have medium pressure.. The rest of the cleaning is just picking and blowing till it's clean on the block, there are no freeze plugs..I have used automatic dish washer soap and water in the system to flush and clean, desolve some in a pail of warm water before pouring it in..

Thanks Tim. I was wondering why I could not find any freeze plugs. I will take your advice. I have a cheapo pressure washer that should do the job. I ordered all my parts today and added to the list the flexible hose in the hydraulic reservoir. I am guessing that is where the oil in front of the radiator was coming from. Also added the valve cover gasket. Bought a borescope and a flushing kit(the kind that goes into the heater hose). Should be able to put it between the coolant heater and the block. I am assuming that the place to drain the block is in the line between the coolant heater and the block since I don't have a petcock.